Enhanced or assisted oil recovery from underground petroleum bearing formations has become a widely practiced technique for extracting a greater percentage of available hydrocarbons from such formations. In present-day practice, steam injection to enhance such recovery is one of the most successful commercial methods. In general the method involves heating water after it has been properly softened, as by treatment in an ion exchanger, in a single-pass steam generator to generate a "low quality" steam (i.e. steam containing a water phase). The resulting steam is then supplied to the earth formation for either stimulating a single well or injected into one well or a plurality of wells as heat and hydraulic pressure to drive oil from the steam injection well to one or more producing wells.
In the production of such steam it is desirable to use any field available hydrocarbons. Frequently a portion of the oil recovered in the oil field is used, rather than fuel transported from a refinery. Where such fuel is available from a refinery, it is generally of a lower quality and frequently contains materials such as sulfur, vanadium and the like which tend to generate corrosion products during combustion. Even with relatively high quality heating fuels, the products of combustion of such fuel may generate corrosion products, particularly in combination with ever present water vapor produced by such combustion. Hence, steam pipes and their support arrangements are subject not only to high temperatures necessary to quickly generate steam in a single, or limited pass, radiant heat section, but also to attack by corrosive combustion products.
A particular form of vapor generator, widely used in enhanced oil recovery, comprises a horizontally elongated furnace having a convection section formed by a horizontal cylindrical shell of substantial diameter, say 8 to 10 feet. The shell supports one or more serpentine coils of pipe formed by running multiple courses of pipes axially parallel to and equally spaced around the circumference of the shell. Steam is generated by water passing through the "coil" formed by U-turns between the pipes at both ends of the cylinder. The cylinder may be on the order of 40 feet in length. The pipes, which may be from 1 to 4 inches in diameter, must be supported radially inwardly from the inner circumference of the cylinder by a plurality of hangers and yokes to provide a stand-off between the steam pipes and the inner wall of the cylinder, including its insulated inner refractory surface, formed by fire brick, or cast.
While in general it is desirable to use a relatively light yoke to avoid loss of heat from it to the shell supported hanger, the size of such yokes has increased progressively to withstand deterioration by corrosion and heat. Attempts have been made to stop such deterioration of the yokes by packing them with insulating material, including casting or wiring (or both) the insulation in place. However, oxidation of parts of the yoke results in the pack not adhering to it over extended operating times. Loss of insulation, of course, results in higher operating costs due to loss of heat through the hanger connection to the outside wall. Further, casting or forming heat resistant coatings around each of several hundred yokes is expensive, both to install and to maintain.
Another solution proposed for such deterioration has been to use more sophisticated steel metallurgy for both the yokes and hangers. Such steels are considerably more expensive than insulated conventional steels if properly insulated. Further, presently operating furnaces could be so retrofitted only by complete reconstruction.
While it is known to use insulating shields or covers for steam tubes or pipes in furnaces and steam generators, such shields are in general intended to be installed permenantly so as to serve as primary supports between tubes. Such shielding means have not been designed for installation or replacement as a field retrofit around existing supports for horizontal, circumferentially distributed, steam generator tubes. For example, U.S. Pat. No. 3,277,872-Hoffmann et al, No. 2,859,737-Banker, No. 2,983,261-Nelson, and No. 3164137-Liessenberg, each disclose shielding arrangements for boiler tubes. However, zone discloses or suggests a replaceable, or retrofit, installation for shielding yokes and hangers to support circumferentially distributed steam tubes that axially extend parallel along the length of a cylindrical steam generator so that they may be serially connected by return loops.